Uniform fuel-air ratio fuel injection system



Feb. 13, 1962 R. B. PEARCE, JR 3,020,717

UNIFORM FUEL-AIR RATIO FUEL INJECTION SYSTEM Filed Jan. 16, 1958 3 Sheets-Sheet 1 FIG.

\ INVENTOR.

RUFUS B. PEARGE,JR.

Bi f/M14 AGENT Feb. 13, 1962 R. B. PEARCE, JR 3,020,717

UNIFORM FUEL-AIR RATIO FUEL INJECTION SYSTEM Filed Jan. 16, 1958 5 Sheets-Sheet 2 FIG.2

INVENTOR.

RUFUS B. PEARCE JR.

WNW 4W Feb. 13, 1962 Filed Jan. 16, 1958 AIR FLOW R. B. PEARCE, JR

UNIFORM FUEL-AIR RATIO FUEL INJECTION SYSTEM 3 Sheets-Sheet 3 VII/II Ir INVENTOR.

RUFUS B. PEARCE, JR.

WM f AGENT Patented Feb. 13, 1962 a 3,020,717 UNIFORM FUEL-AIR RATED FUEL lNJlECTlON SYSTEM Rufus B. Pearce, 5n, Fulierton, Calif, assignor to North American Aviation, line. Filed .lan. l6, 195$, Ser. No. 709,418 8 Claims. (Cl. oil-39.28)

This invention relates to systems which inject fuel into power plants.

More specifically, this invention relates to a fuel in jection system for power plants, typically of the jet engine type, which have a plurality of fuel injection nozzles, the flow through which may be independently varied in order to compensate for uneven air distribution in the engine duct.

In our present high-performance missiles which use ramjet or turbojet engines as the primary power plant, a problem exists in obtaining the correct fuel-to-air ratio over the entire cross-section of the duct of the engine and thereby insure proper combustion. Until now it took a great deal of engine design to shape the duct of the engine to obtain nearly a uniform airflow across the entire cross-section of the engine duct. In order to accomplish such uniform airflow, certain sacrifices from optimum design had to be made with a resulting lower efficiency. The present invention provides a plurality of fuel injection nozzles as has already been proposed in the past; however, the present invention provides a sensing means immediately in front of each of the nozzles to sense the rate, by weight, of the airflow passing that nozzle and this sensing device controls a trim valve in the fuel line leading to that particular nozzle to increase or decrease the fuel flowing to that nozzle to correspond with any increase or decrease in the weight of air passing around that nozzle. With such a control which is able to inject different amounts of fuel through the different nozzles in the duct, the duct may be designed for optimum efficiency and obtain a uniform fuel-air ratio at the engine face without requiring that a uniform cross-sectional airflow be obtained.

The present invention also is used in the after burner of jet engines where additional fuel is injected into the gases leaving the jet engine burner chamber for additional thrust.

Although the largest use of the present invention by the assignee is in jet engine fuel systems, the spirit of the invention is also useful in almost any situation where it is desired to inject some matter such as a gas, liquid spray or fluidized solid particles into a stream of fluid such as liquid or gas to obtain a uniform mixture of the first and second materials. For example, the invention could be used to inject powdered coal into air flowing to a combustion chamber of a furnace and obtain a uniform fuelair ratio across the cross-section of the inlet passage.

Therefore, it is an object of this invention to provide a means of producing uniform fuel-oxidizer ratio across the cross-section ofa duct carrying oxygen.

It is a further object of this invention to provide a means which will sense the weight of air passing a fuel injection nozzle within a jet engine duct and control the fuel flow through that nozzle in proportion to the weight of airflow around the nozzle.

It is a further object of this invention to provide a secondary trim control which will modify the fuel flow through a plurality of fuel nozzles to insure a uniform air-fuel ratio across the cross-section of an engine duct.

Other and more specific objects will become apparent in the detailed description below, wherein FIG. 1 is a schematic of the control system in a jet engine;

FIG. 2 shows a schematic cross-section of the jet engine duct with the fuel control system;

FIG. 3 discloses the primary and trim controls of the system in detail; and

FIG. 4 discloses an alternate means of sensing the rate of flow of air in a jet engine duct.

FIG. 1 shows fuel injection nozzles within a series of pods 5 which are located Within the duct bonded by the walls 2 of the jet engine and are located down stream of the diffuser 3 and up stream of the flame holders 4 which are at the engine face. The pods could be used to inject fuel into an afterburner also, and therefore the phrase within the engine duct includes that portion of the duct which is in proximity to the afterburner of an engine. The pods 5, which contain the individual air measurement means and the fuel nozzles, are denoted as A through E for the five pods which are shown in FIG. 1 and FIG. 2. It is to be understood that this number of pods is merely illustrative and any number of pods may be provided. As shown, air pressure lines 6 lead from the respective pods to the distribution mechanism 7, shown in block form, containing the primary and the trim controls which will 'be explained in more detail below. Leading from the mechanism 7 are individual fuel lines 8 which conduct the fuel to the individual nozzles. As explained in connection with FIG. 3, each of the pods 5 for the specific embodiment shown contain a choked orifice and therefore, choked orifice outlet lines 9 are provided. For purposes of clarity, all of these above mentioned lines are shown as passing through the bottom portion of the duct wall 2, but it should be understood that in practice, the lines would lead to the portion of the wall 2 which is closest to the respective pod as shown schematically in FIG. 2 and would be enclosed in the supporting structure for the pod such as the pylon shown in FIG. 3 or FIG. 4. A pump 11 supplies the fuel to the distribution mechanism 7 through main fuel line 12 from a fuel source, not shown, such as a fuel tank.

FIG. 2 is a schematic drawing of the fuel distribution system looking aft from the diffuser showing the pods with the choked orifice to be disclosed in detail below. The pods are shown as being located so that each will cover approximately one-fifth of the cross-section of the engine duct, but the invention is not limited to the exact location or number of the nozzles. As explained below, the nozzles may be positioned anywhere across the engine face in the duct if proper design adjustments are made.

FIG. 3 illustrates the invention in detail, by means of two subsystems A and B, each of which are controlled in unison by a primary control and each individually further controlled by the secondary trim control hereafter described. It is to be realized that other subsystems leading to any other individual nozzles would be connected in the same manner.

The pump 11 brings fuel from fuel tank or reservoir, not shown, through the main fuel line 12 and introduces it to the manifold 14 which conducts the fuel to the respective subsystems. The fuel then is passed through primary and secondary valves 20 and 21, respectively, in each of the subsystems; the valves controlling the fuel flow into the jet engine duct. Primary control 16 reprecents any one of the standard fuel controls which is commonly known in the art such as shown in Patent No. 2,512,790 or Patent No. 2,796,730. Such automatic controls automatically control the overall fuel flow to a jet engine in order to keep a constant vehicle Mach number or to keep the normal shock wave in the diffuser section of the engine duct when the air vehicle is operating in the supersonic range. The control 15 could also be worked by a human pilot if the present invention were used in manned aircraft.

This control 16 through operative shaft 17 and cross bar 18, actuates valves 20 to control the overall fluel flow to the engine through all the subsystems.

With the control linkage system shown, the valves 20 will all be shaped identically. If the control linkage is such that the operative element 22 of one primary valve is moved more than another, the operative element 2?. of the respective valves are shaped to compensate for the difference in motion.

To describe the secondary trim control, only the control used in subsystem A will be described since the trim control in subsystem B is identical. Secondary or trim valve operative element 24 is located juxtarelated to trim valve seat 25 which is mounted to the walls of fuel line 8a. The position of the trim valve is controlled by the pneumatic trim control unit indicated generally by arrow 27. The unit 27 consists of housing 28 which has a rigid partition 29 mounted therein which divides the housing into two compartments. A shaft 30 is movably mounted in said housing and passes through the partition 29 and one wall of the housing and into the fuel line 8a, as shown. This shaft has said trim operative valve element 24- mounted on it. Thus, the position of shaft 3% controls the opening of the trim valve 21. Appropriate O ring seals 33, as shown, are provided to allow the shaft to pass through the various obstructions without leakage. Flexible diaphragm 31 and flexible diaphragm 32 are mounted in the two compartments respectively to the walls of the housing and, as shown, are substantially in a transverse relation to the longitudinal axis of the shaft 30 forming four chambers within the housing 28. As set out below, these diaphragms 31 and 32 are sized to obtain a certain ratio between their operative areas A and A in order to obtain the best operation. The operative area of a diaphragm is the area which receives the force of the pressure in the adjacent chamber and transmits this force to the shaft 30. While the overall size of the diaphragms is not critical, it should be at least ten times larger than the opening of the secondary valve for best control. The diaphragrns may be of the same or different diameters, depending on the ratio required but with diameters of the order of two inches the trim control unit 27 will work satisfactorily. The shaft 30 is connected to the flexible diaphragms 31 and 32 so that any pressure on the operative areas of these diaphragms will tend to urge the trim valve 21 open or closed as required. A connecting conduit 34 is provided so that the chamber to the left of flexible diaphragm 31 is subjected to the pressure Pm between the fuel pump 11 and the primary valve 29 which tends to urge the secondary valve closed. Connecting conduit 35 is provided to connect the chamber on the right side of flexible diaphragm 31 to the restricted pressure Pr between the primary valve 29 and the secondary valve which tends to urge the secondary valve open. The chamber on the righthand side of the flexible diaphragm 32 is connected by conduit 6a to the air measurement means described in more detail below so that any increase in the air pressure will tend to urge the trim valve to open. The chamber on the left side of the flexible diaphragm 32 may be subjected to any standard reference pressure as desired; however, it has been found that if the conduit 36 is connected to a standard constant vacuum source, the design problems are easier as indicated below. If the operative element 24 were shaped so that the trim valve 21 is closed as the shaft 30 is moved to the left, then the conduits 34 and 35 leading into the chamber would be reversed and conduits 36 and 6a respectively would be reversed. Thereby, the force of pressure Pm on A and F on A would still urge the trim valve closed and the force of pressure Pr on A and P2,, on A would still urge the trim valve open.

The housing may be made of formed sheet metal which is welded together, but the most feasible method is to form it of stacked layers of metal as shown, that is,

a layer of the proper thickness is provided for the two ends of the housing, for each of the chambers, and for the partition 29. These layers are drilled and machined as required and then they are stacked as shown in FIG. 3 and held together with some means as bolting (not shown). This latter method of manufacture which is commonly known in the hydraulic and pneumatic control field, has the additional advantage that the size of the layers of metals can be made large enough so that all of the desired trim control units can be side by side in one stack thus facilitating manufacture, and saving weight and space.

FIG. 3 also shows an enlarged view of one configuration the pod, which is designated So because it is used with subsystem A, can take. Only pod 5a and its internal structure is shown as pod 5b which is provided for subsystem B and any other subsystems would be identical. An air measurement means indicated generally by arrow 37 must be provided associated with each fuel nozzle to sense uneven airflow distribution in the duct. Therefore, at its leading section, the pod 5a is provided with a choked orifice 38 having a static pressure tap 39 mounted therein which measures the static pressure of the air passing through the orifice. Outlet line fizz, as shown, conducts air out of the chamber behind the choked orifice 38 either to atmosphere as shown or to a vacuum source so that there is a continual flow through the orifice. The static pressure measured indicates the weight flow of air through the orifice (e.g. pounds/sec.) and since this flow is a sample of the airfl w in the duct near each of pods 5, the pressure measured is an indication of airflow into which the nozzle will. inject fuel. If the pod is used in the afterburner of an engine, then the pod may be made of ceram c in order to withstand the temperatures involved.

At the rear of pod 5a is located fuel nozzle 49 which has fuel line 8a leading into it. This fuel nozzle is of any shape desired in order to give optimum spray characteristics. If each of the pods 5 in a particular engine fuel system is located within the duct to cover the same area of the duct as the other pods used in conjunction with it, all of the nozzles will be of the same shape and size; however, when desired, the nozzle is made smaller or larger and the spray path is changed in order that each nozzle will cover a particular portion of the duct.

FIG. 4 shows another arrangement for sensing the flow of air around each nozzle. The air measurement means 37 here take the form of a Pitot tube 42 which is provided extending forwardly in the duct from the pylon 43 which supports the pod S in the duct. This Pitot tube 42 measures the total pressure of the air stream at its open end which is an indication of the total pressure of, and consequently the rate of flow by weight of the air stream in proximity to the particular pod 5. This is because the total pressure of the stream is a function of the weight flow of air in the stream. A fuel nozzle 4 is provided at the rear end of the pod 5 and is shaped to give the spray desired in the same manner as nozzle 40. As shown, the fuel line 8, connected to the nozzle -44, is provided and leads to primary and secondary control valves such as described above.

In order to have minimum errors the air pressure measurement means for each of the subsystems should all be the same, e.g., all Pitot tubes, or static pressure taps, etc. The above specific embodiments are shown because they are the simplest and consequently the most reliable means of sensing the flow of air past each particular nozzle, but the spirit of the invention is not limited to the structure shown.

As mentioned above the ratio of the operative areas A and A of the diaphragms is critical; therefore, the following description is provided to teach the method of obtaining this ratio. In order to facilitate the description it will be assumed that all of the primary valves move together the same distance when actuated by the primary control and that each of the fuel nozzles is placed to cover the same amount of cross-section of the duct as the other nozzles.

The desired fuel-air ratio K for subsystem A and all other subsystems is shown by the following equation As stated above, in order to simplify the design, the P is a vacuum so that the equation takes on the form PmA Pr A Pt A =O As mentioned above, the airflow measuring means 37 may take on any one of a number of forms; however, with the Pitot tube or the choked orifice specifically set forth the total pressure or static pressure measured is an indication of the amount of air passing around the pod. This is because the pressure measured is a function of the amount of weight flow through the choked orifice and the relationship between this latter weight flow and the weight flow through the cross-sectional duct area into which the respective nozzle is designed to inject fuel is known. Therefore, the following equation is true.

KPt

1/ T where Wa is the weight flow of air in, for example, pounds/ second passing around the pod in the duct crosssectional area into which the nozzle is designed to inject the proper amount of fuel to produce the desired fuel-air ratio. Pt is the static pressure which is measured near the choked orifice 38 or the total pressure measured by Pitot tube 42. T is the total temperature of the air stream, and K is a constant which relates the weight flow of air into which the particular fuel nozzle is injecting fuel and the pressure measured at choke orifice 38 at any temperature. The value of K is approximately the product of the normal constant for determining the weight flow through the choked orifice times the ratio of the duct cross-sectional area that the particular fuel nozzle is designed to cover to the minimum area of the choked orifice 38 or the cross-sectional area of the Pitot tube 42. Thereby, it is seen that by continuously measuring Pt an accurate measurement of the weight of air passing through the area into which the particular fuel nozzle is injecting fuel at any instant is made, and by applying this pressure Pt to the control system, the fuel injection rate may be varied to obtain the desired fuel-air ratio.

Equation 5 may be rewritten as Wa fi -T The fuel flow rate through the fuel line 8a is determined by the following equation where W) is the rate of fuel flow through the subsystem A; A is the area of the opening of the primary valve 20; )(e) is a function of the particular valve coordinating the fuel flow with the area of the valve, and (PmPr,,) is the pressure drop across the primary valve. Pm is the 6 fuel pressure between the primary valve and the pump and Pr is the fuel pressure between the primary valve and the secondary valve in subsystem A.

For the reason explained below, it may be assumed that the area of the primary valve opening is constant. Briefly, this is because the subject invention provides a means of adjusting the fuel flow through each particular nozzle to obtain a uniform fuel-air ratio from all of the nozzles even though the primary valve is not varied. Therefore, the equation takes on the form where K is a constant for the valve for coordinating the fuel flow through the valve with the pressure drop across the primary valve at that particular valve opening.

Equation 8 may be written as By substituting Equations 6 and 9 into Equation 4, the following equation is obtained since the air-fuel ratio is assumed to be K by Equation 1, the following equation results evenly across the cross-section of the duct, that is, each nozzle is located in order to inject fuel into the same size cross-section of the airflow in the duct. In this case, all of the orifices are matched so that the value of K for any other subsystem would be equal to the value of K for subsystem A. Likewise, since it is desired to have a uniform fuel-air ratio, the value of K for any other subsystem is equal to the value of K for subsystem A. Also, as stated above, the value of K for each primary valve relates the flow of fuel through the valve to the pressure drop across the valve at any particular valve opening. Therefore, the valve of K is determined by control 16, and since all of the primary valves 20 are assumed to be identical in configuration, the value of K will be identical. Further, the total temperature of the air stream passing through the duct cross-section is substantially constant at any instant. Therefore, Equation 13 is true for all of the subsystems. It is true that K will become larger as the area of the particular primary control valve is increased, but since the fuel-air ratio and consequently K will also increase, the equation remains true. Also, it is true that the total temperature may vary during the flight. Therefore, diaphragms are designed with the above ratio when the system is at the point at which it will be operated most of the time to decrease errors when. the subsystems are operated above or below the average point. Also, it should be remembered that the primary valve can compensate for these errors since they will occur across the whole cross-section of the duct.

If it is desired that certain of the nozzles inject fuel into a larger area of the duct than other of the nozzles, the particular subsystems to these nozzles is made larger, thereby more fuel will pass through this subsystem with the same pressure drop across the particular larger primary valve which would have a value of K which is larger than the other values at any particular setting of the primary control 16. Referring to Equation 12, it is seen that K can remain the same for the different subsystems by increasing or decreasing the value of K to correspond to an increase or decrease in the value of K. As stated above, K takes into account the size of the cross sectional area of the duct into which fuel is being injected into in relation to the size of the choked orifice.

In operation, an automatic control system which is represented by primary control unit 16, controls the overall amount of fuel passing through the manifold system and the setting of the primary valves 20 dictates the overall fuel-air ratio in the engine duct. Assume for an instant in the system shown in FIG. 3 the primary valve has reached its steady state condition and the area openings of the primary valves 2!} are constant, and then assume the weight flow of air in the area of pod 5a increases. This increase is sensed by the pressure tube 39 which increases the pressure Pt on the right-hand side of flexible diaphragm 32 urging the secondary trim valve open further. However, this will cause the pressure between the primary and secondary valves Pr to decrease as the pressure drop across the secondary valve will be less. This causes less pressure on the right side of flexible diaphragm 31 and therefore, there is less force resisting the force of pressure Pm on the left side of flexible diaphragm 31 which always tends to urge' the trim valve closed. Therefore, it is seen that the increase in the pressure Pt, moves the operative element 24 to the left and consequently move the trim valve open until the pressure Pr drops enough that the force of pressure Pm on A balances the force of pressures of Pr and Pt on A and A respectively. At this new trim valve condition, more fuel will flow through the lead line 8a for the same primary valve opening since the pressure drop across the primary valve is increased. Thus, it is seen when there are any changes in the airflow in the engine duct, the subsystem automatically makes a change which will change the flow of fuel to the proper nozzle to retain the appropriate fuel-air ratio which primary valve 20 continues to dictate.

It is also seen that any changes in the fuel pressure in front of the primary valve Pm within the compensating ability of the particular trim valves will not affect the amount of fuel passing through the subsystem. If pressure Pm increases, the trim control unit 27 becomes out of balance and the trim valve will be urged closed which causes the pressure Pi to increase until the force of the latter on the righthand side of flexible diaphragm 31 is enough to balance the force of the increase pressure Pm on the left side of the flexible diaphragm 31. With the trim valve thus more restricted, the pressure drop across it will be more and the increased pressure Pm will be reduced to its proper level down stream of the trim valve and consequently at the nozzle.

The detailed analysis of the invention has been described above in conjunction with a jet engine fuel system having only two nozzles and airflow measuring means in the engine duct for simplicity of description. It should be understood that any number of subsystems with their nozzles could be used and that the primary valves in the different subsystems could be designed to obtain any specific results desired. Further, it should be understood that the invention is not limited to fuel systems but could be used for other things such as injecting small particles into a flow of water or liquid chemical into a stream of air. The main object of this invention is to provide a trim control for an injection system which is able to compensate for the fact that more ambient fluid may flow past one injection nozzle than another. Therefore, although only one specific embodiment in conjunction with a jet engine has been shown and described, it is within the skill of the art to vary the fluids involved, the means of sensing the ambient fluid flow, and the means of trimming the flow of the injected fluid described above and the 8 appended claims should not be limited to the specific embodiment shown.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. Means for controlling the distribution of fuel being injected into a fuel-air burning power plant having a combustion chamber, said means comprising a plurality of fuel nozzles in substantially transversely spaced relationship within the inlet passage to a combustion chamber of a power plant, said nozzles substantially constructed and arranged in a plane which is at right angles to said inlet passage; fuel conducting means, means communicating with said fuel nozzles through said conducting means for controlling the overall amount of fuel to be injected into said inlet passage through said nozzles, means adjacent each of said spaced nozzles to sense uneven air distribution in said passage, and means responsive to each of said last mentioned adjacent means for secondarily controlling the fuel flow to their respective adjacent nozzles individually to compensate for said uneven airflow through said inlet passage so that burning occurs at a uniform fuel-air ratio.

2. Means for controlling the distribution of fuel being injected into a jet engine, said means comprising a plurality of fuel nozzles in substantially transversely spaced elationship within a jet engine duct and substantially in a plane which is at right angles thereto, valve means for controlling the overall amount of fuel to be injected into said duct through said fuel nozzles, means adjacent each of said spaced nozzles to sense uneven air distribution in said duct, secondary valve means responsive to each of said last mentioned adjacent means for secondarily controlling the fuel flow to their respective adjacent nozzles individually to compensate for said uneven airflow through said duct, and fuel conducting means connecting said valve means, said secondary valve means, and said fuel nozzles so that burning occurs at a uniform fuel-air ratio.

3. A jet engine fuel control system comprising a duct and a plurality of fuel injection nozzles substantially transversely spaced at right angles to said duct, primary control means for principally controlling the overall fuel rate through the nozzles, sensing means related to at least two of said nozzles, each of said sensing means for producing an output which is a function of the rate of flow by weight of the air in the duct in proximity to each of said respective nozzles, secondary trim control means responsive to the output of each of said sensing means for modifying the fuel flow to each nozzle having said sensing means related thereto as a function of the output of the respective sensing means, and fuel conducting means connecting the primary and secondary control means and each of the respective nozzles.

4. In combination with a jet engine duct, a fuel system comprising a plurality of fuel injection nozzles in spaced relationship located in a plane at right angles to the axis of the said engine duct, airflow sensing means for sensing the weight of air flowing around each individual nozzle, said sensing means including an air pressure tap juxtarelated to a choked venturi orifice, a source of pressurized fuel, primary control means connected to said fuel source for controlling the overall rate of fuel flow through said nozzles, a secondary trim control means connected to and between said primary control means and each individual nozzle for controlling the rate of fuel fiow to each of said nozzles individually, each of said trim control means being connected to and at least partially responsive to the pressure measured at a respective pressure tap.

5. In a jet engine duct, a fuel system comprising a plurality of fuel injection nozzles in spaced relationship located in a plane at right angles to the axis of the said engine duct, a source of pressurized fuel, fuel conducting means having a plurality of fuel lines connecting said pressurized fuel source to each of said nozzles, primary control means connected in said fuel conducting means for controlling the overall rate fuel flow through said nozzles, a secondary trim control means connected in each of said fuel lines and between said primary control means and each individual nozzle for varying the rate of fuel flow to each of said nozzles individually, airflow means for sensing the rate by weight of the air flowing in the duct in proximity to each individual nozzle, said airflow means including an air pressure tap juxtarelated to a choked venturi orifice located within the duct near each of said nozzles, each respective trim control means being connected to and at least partially responsive to the pressure measured at each respective air pressure tap.

6. The invention as claimed in claim wherein at least one of said trim control means includes a valve having a movable operative element and valve seat within one of said fuel lines, and diaphragm means responsive to the pressure measured at the respective air pressure tap for controlling the position of said operative element in relation to said valve seat.

7. The invention as claimed in claim 5 wherein each of said trim control means includes: a housing, a shaft movably mounted in said housing and passing one wall of the housing, said shaft extending into one of said fuel lines, a trim valve seat within said last mentioned fuel line, said shaft having an operative element of a trim valve within said fuel line for opening or closing the trim valve by movement of the shaft, a flexible diaphragm mounted to the walls of said housing and substantially transversely to the shaft forming two chambers in said housing, said flexible diaphragm connected to said shaft whereby movement of the diaphragm moves the shaft and the trim valve operative element, and one of said chambers communicating with the air pressure tap located near the nozzle to which said last mentioned fuel line is conducting fuel.

8. A fuel injection system comprising a fuel pump, a plurality of fuel injection nozzles, air pressure measuring means mounted on each of said nozzles for sensing the rate by weight of the flow of air passing around each respective nozzle, fuel conducting means having a plurality of fuel lines connecting said pump with each of said nozzles, primary control means including a primary valve for variably restricting the overall flow of fuel through the fuel conducting means, a secondary trim control means connected in one of the fuel lines of the conduct ing means for restricting the flow of fuel to the respective nozzle, said trim control means including a housing divided into two compartments by a fixed partition, a shaft movably mounted in said housing and passing through the partition and one wall of the housing, said shaft extending into said last mentioned fuel line, a trim valve seat within said last mentioned fuel line, said shaft having an operative element of a trim valve within said fuel line for opening or closing the trim valve by movement of the shaft, a flexible diaphragm mounted in each of said compartments to the walls of the housing and substantially transversely to the shaft forming first, second, third, and fourth pressure chambers with the housing and the partition, said first chamber being connected to the fuel pressure between the pump and the primary valve and exerting a pressure on one of the flexible diaphragms tending to urge the trim valve closed, said second chamber being connected to the fuel pressure between the primary and trim valves and exerting a pressure on one of the flexible diaphragms tending to urge said trim valve open, said third chamber containing a predetermined pressure, and said fourth chamber being connected to said air pressure measuring means and exerting a pressure on one of said diaphragms tending to urge said trim valve open.

References Cited in the file of this patent UNITED STATES PATENTS 1,363,513 Keith Dec. 28, 1920 2,865,170 Kadosch Dec. 23, 1958 2,869,322 Rankin Jan. 20, 1959 FOREIGN PATENTS 17,813 Germ-any Ma 24, 1956 762,179 Great Britain Nov. 28, 1956 765,359 Great Britain Jan. 9, 1957 

